1. Field of the Invention
The present invention relates to a high frequency or microwave parallel strip line cable comprising a connector part, and a connector provided on a dielectric: or semiconductor substrate for connecting with the connector part, which are utilized at a frequency band higher than about 800 MHz.
2. Description of the Related Art
Conventionally, as a high frequency or microwave transmission line cable for transmitting a high frequency or microwave signal, a coaxial cable 53 has been widely used which comprises a center conductor 50 buried in a cylindrical dielectric body 51 and an earth conductor 52 of braided wires formed on an outer periphery of the cylindrical dielectric body 51, as shown in FIG. 8 which shows a connection structure between a conventional connector set 60 and the conventional coaxial cable 53. The connector set 60 is provided for connecting with the coaxial cable 53, and comprises a pair of male and female connectors 61 and 62, wherein the male connector 61 is mounted on an end of the coaxial cable 53, and the female connector 62 is formed on a printed circuit board or dielectric substrate 20.
When the coaxial cable 53 is inserted into the male connector 61, the coaxial cable 53 is connected with the male connector 61 so as to be fixed therein. Thereafter, the male connector 61 is inserted into the female connector 62, and fixed therein, thereby electrically connecting the coaxial cable 53 to a circuit formed on the printed circuit board 20.
Accompanying recent down-sizing of electronic units, a demand for down-sizing, in particular, lowering of thickness of connection structures of high frequency or microwave line cables mounted in spaces of inner parts of the electronic units has been heightened. However, the above-mentioned connection structure between the conventional coaxial cable 53 and the conventional connector set 60 leads to such a problem that it is extremely difficult to lower the thickness thereof for the following reasons.
A characteristic impedance Z.sub.0 of the coaxial cable 53 is represented by the following equation (1): ##EQU1## where
D is an inner diameter of the earth conductor 52 of the coaxial cable 53;
d is an outer diameter of the center conductor 50 of the coaxial cable 53; and
.epsilon.r is a dielectric constant or a relative permittivity of the dielectric body 51 of the coaxial cable 53.
The center conductor 50 of the coaxial cable 53 is made of a solid wire or stranded wire, and therefore, the minimum limit of the outer diameter d of the center conductor 50 may be about 0.1 mm because of the manufacturing method thereof. When the dielectric body 51 made of fluorocarbon resin having a dielectric constant .epsilon.r of 2.04 is formed around the periphery of the center conductor 50, the minimum limit of the inner diameter D of the earth conductor 52 becomes about 0.33 mm as apparent from the above equation (1).
Further, since the earth conductor 52 is generally made of braided wires, at least about 0.2 mm is required for the thickness of the earth conductor 52. Furthermore, when taking into consideration the thickness of an outer sheath 54 for covering the earth conductor 52, at least about 0.73 mm is required for the outer diameter of the coaxial cable 53.
As mentioned above, there is a limit of down-sizing of such a coaxial cable 53, and the size of the high frequency or microwave transmission lines mounted in the spaces in the inner portions of the electronic units can not be further decreased, in particular, the thickness thereof can not be decreased so as to satisfy the above-mentioned demand.
Furthermore, there is a limit to the possible amount of reduction in size and thickness of the connector set 60.